Protective device



Nav. 9 1926. 1,606,143

'J. T. BUTTERFiELD PROTECTIVE DEVICE Filed April 11. 1923 /l/yz/mfa L/mf Kufer/75% Patented Nov. 9, 19.26.

UNITED STATES Partnr JOSEPH 'i'. BUTTERFIELD, OF HAWTHORNE, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEVI YORK, N. Y., A CORPORATON OF NEW YORK.

PROTECTIVE DEVICE.

Application filed April 11, 1923.

- This invention relates to protective devices and particularly to a fuse suitable for protecting delicate apparatus.

The object of the invention is in general to provide a reliable, sensitive fuse suitable for protecting very delicate electrical apparatus.

lt is common practice to protectelectrical apparatus against excessive current by inio serting in circuit with the apparatus a fuse element which consists essentially of a fusible conductor of low melting point and of such a size that it will be melted by a lessery current than Will damage the apparatus. The familiar forms of fuse elements which are available for protecting relatively rugged apparatus are not, however, suitable for protecting very delicate apparatus. The reason for this is threefold. Firstly, in order to protect some very delicate apparatus, the fusible conductor would have to be of a size which is smaller than it is practicable to manufacture and handle. Secondly very small fuse Wires when used in the usual types of fuse elements may oxidize so rapidly under service that the fuse will soon blow Without the passage of an excessive current. Thirdly, even when a single lot of fuses are manufactured with the greatest 3o care to obtain uniformity, many of them will be blown only by a current Which is considerably larger than the rated capacity of the fuse; and there lis no suitable means for testing the individual elements Without destroying them.

Some moderately sensitive apparatus can be fairly Well protected by fuses made of gold, which can be formed into quite small conductors and does not oxidize. However d20 it is impractical to protect very sensitive apparatus even by gold fuses and these, in i common With other known fuses, cannot be individually tested Without destroying them.

ln accorcance With the general features of the present invention, a fuse element is constructed with a very fine fusible Wire and is rendered sensitive and reliable by placing the fusible Wire in an evacuated chamber.

Referring to the drawings, Fig.` l shows a convenient form of fuse elements constructed in accordance with the present in- Serial No. 631,356.

vent-ion and Fig. 2 shovvs schematically a circuit in which delicate current measuring apparatus is protected by a fuse constructed in accordance With this invention.

A fine Wire l which may be of any suitable material is mounted in a Well lrnovvn manner and enclosed in an evacuated chamber 2.

Leads from the fuse Wire may be connected melt at a lower current than Would be required if the Wire Were exposed to the atinosphere.

This invention contemplates the use of any suitable material for fuse Wire 1, but attention is called to an alloy of approximately copper and 0% nickel, Which may be obtained commercially under the trade name of Advance Many Well known fuse alloys containing lead, tin, and bismuth, have a lovver melting point than this alloy, but it is difficult, if not impossible to reduce a Wire of such alloys to` a size which will give protection to very delicate apparatus. Advance Wire has the apparently detrimental characteristic of a higher melting point than many of the so. called easily fusible alloys, but this is offset by its high resistance Which causes more heat to be developed in the Wire.

An additional feature of this invention is the making of the fuse wire from an alloy having a substantially Zero temperature coefficient of resistance. ln many instances it is of no importance Whether the resistance of a fuse changes Wit-h temperature or not, but in other cases especially Where measurements are being made with very small currents, it is frequently important that the conditions of the circuit remain constant. lt is apparent, therefore, that if the resistance of a fuse changes With temperature, its resistance may vary considerable for different values of current, since fuses are subjected to relatively high current densisties.

The alloy lrnovvn as Advance is one of several known alloys Which have a substantially Zero temperature co-eliicient of resistance.

In Fig. 2 the fuse is shown in circuitl to protect a thermocouple 3 which, in conjunction with galvanorneter G measures the alternating current induced in the circuit i by means of transformer 5 and source of current 6. Fuses of this type have been found to be Very effective for protecting therinocouples which are damaged by currents in excess ci 25 inilliainperes; and can be constructed to protect even ino-re delicate apparatus. ther obvious uses to which the fuse may be put are the protection. of delicate galvanoineters and the 'vibrators of osciliographs.

uses constructed in accc-rdance `ritli the present invention are not damaged by a current 'v-hich just causes thein to gleuf; and for any given design ef iuse there is a definite relation between the Value of current vzfhich justcauses each individual fuse to glow, Yand the current. which Will cause that :fuse to open the circuit. This enables each 'use te be tested-to determine its proper rating, thus avoiding the uncertainty Which exists when therating is determined by the vperformance 0I" other specimens.

What is claimed is:

l. An electric fuse consisting of a lilament of an alloy of approximately 60% copper and 4;() U75 nickel enclosed in an evacuated vessel.

2. An electric fuse comprising a tine Wire or lilament of highly ductile Inetal enclosed in an evacuated vessel and capable of operating to break the circuit in which it is included on a current of less than .O5 ampere. j

An electric fuse cen'iprisine` a filamentary vWire composed vo'l metal or practically Zero coeliicient of resistance.

et. fin electric fuse of such iinely drawn wire tiiatiit is capable oi operating to break a circuit inA which it is included on a current el ies-s than dyniilliainperes.

5. An electric fuse consisting of a fine inctal ilanientcapable of withstanding a glewingtemperature `without breaking down but effective to kopen 4the circuit in which it. is included on a lcurrent of less than inillianiperes.

in witness ywher-echi hereunto subscribe my this 10th day of April A. D., 19237.

ill OSEPH T. BUTTERFIELD. 

